We have investigated properties of chemically modified boron nitride nanotubes (BNNTs) with NH3 and four other amino functional groups (NH2CH3, NH2CH2OCH3, NH2CH2COOH, and NH2COOH) on the basis of density functional theory calculations. Unlike the case of carbon nanotubes, we found that NH3 can be chemically adsorbed on top of the boron atom, with a charge transfer from NH3 to the BNNT. The minimum-energy path calculation shows that a small energy barrier is encountered during the adsorption. Similarly, a small energy barrier (about 0.42 eV) is also involved in the desorption, suggesting that both adsorption and desorption can be realized even at room temperature. For chemically modified BNNTs with various amino functional groups, the adsorption energies are typically less than that of NH3 on the BNNT. The trend of adsorption-energy change can be correlated with the trend of relative electron-withdrawing or -donating capability of the amino functional groups. Overall, the chemical modification of BNNTs with the amino groups results in little changes in the electronic properties of BNNTs. However, the chemical reactivity of the BNNTs can be enhanced by the chemical modification with the amino group containing -COOH.